In a wicket gate system of a hydroelectric turbine, the gates are oriented by means of a mechanism having articulated arms. The articulations of the arms are maintained by shear pins adapted to break when an abnormal effort is applied on a gate. Such a situation may happen if, for example, a gate jams a wood log or a rock during the closure. By breaking, the shear pin protects the gate and all the gating mechanism which is linked to it against more important breakage. Each shear pin acts so to speak as a mechanical fuse which protects the gating mechanism. It is necessary to detect as soon as possible the breaking of a shear pin and to identify the defective shear pin to re-establish the proper functioning of the gating system. For this purpose, it has been taught to insert a tearing or breaking detector in each shear pin.
The current detectors are normally made of a wire inserted in a bore at the center of a shear pin. The bore is filled with epoxy to hold the wire in place. Each detector is connected in series with the others up to a common alarm point usually supplied with less than 100 Vac.
When a shear pin breaks, the wire is cut and the loss of power supply triggers an alarm. The detectors being connected in series, it is not possible to readily determine which shear pin is defective.
By their situation, the wires connecting each detectors are sometimes snagged and disconnected. In order to limit the reparations, each detector is preferably provided with quick connectors. If a cable breaks, only the broken part is changed. Mechanical protection systems are also added on each shear pin to protect the quick connectors against breakage by snag. Despite everything, certain mechanical breakages requiring replacement of the cables occur. Furthermore, the shear pins often bathes in water, oil and grease. In the long run, these conditions finally attack the cables and the connectors.
There is often groundings and the detection is difficult since the system is connected in series, in floating point. Since the wires are embedded in epoxy, it is then necessary to remove the shear pin completely, to clean it from the epoxy and re-install a new detector. Faced with the extent of work to be done, it becomes more simple to change the shear pin by a new one, which involves higher costs. In the case where the connections between the detectors run from the bottom of a shear pin to the top of another, it may be difficult to remove the portion of a broken cable hanging under a shear pin.